Rear Diff Lock Installation Guide

Note: Read entire guide before starting installation.

Use common sense! This is a simple procedure. Don't do something if it doesn't seem safe or doesn't make sense to you. If in doubt, stop and seek advice from a qualified mechanic or ask for advice.

Background: As part of the original VW Vanagon Syncro design, a functionally identical device to this one was installed on those vehicles that came with a Front Diff-Lock (this was not a US option). The primary motivation was one of safety. If the front-diff knob is inadvertently pulled out to engage the front diff-lock (such as a child might do when the van is parked), the first indication to the driver (assuming he doesn't notice that the knob is out) is the front-diff dash light coming on simultaneously with the steering wheel abruptly pulling to the left or right. It will be almost impossible to steer, and if you are on pavement, you will most likely break a front axle or CV joint. This might happen just as you are pulling out of your driveway, or it might happen at 70 MPH as you get to that first big curve on the freeway. The idea is that a blinking dash light might let you know so that you can disengage it before starting your drive.

This is not a terribly complex device. It consists of a switch that hooks up to the vacuum line that connects to a circuit board. This combination of devices is able to blink the dash light when there is sufficient vacuum available at the diff-lock switch to engage the diff-lock, prior to engagement where the light comes on full.

This device provides some diagnostic functions, as well as it's primary purpose as a safety warning device. If there is a problem with the vacuum connection between the engine and the diff-lock switch, then there is not enough vacuum to either blink the light or engage the diff-lock (time to check the lines between the engine and the vacuum reservoir up to the dash). If the blinking light doesn't immediately come on, but starts blinking after a bit, and then engages, you likely have a problem with vacuum line on the dis-engage side of the diff-lock unit (between the dash switch and the control unit). If the blinking light comes on but the diff-lock does not engage as it should, then the diff-lock vacuum control unit at the differential should be checked. If neither the blinking light comes on nor the solid light comes on once engaged, then there is likely a bad bulb in the dash. If the blinking light comes on, but it just keeps blinking when the diff-lock has engaged, then there is a problem with the screw-in switch on the differential.

Although the danger of accidentally engaging the rear diff-lock is usually not so catastrophic, it can still be a nasty problem on pavement, and could cause some damage to the transaxle and/or axles. The diagnostic capability that it provides can reduce diagnostic time. And there seems to be a certain comfort for a Syncro driver when they can see a pending diff-lock engagement.

Parts List:
 Vacuum Sensor Switch connected to rubber hose and Y connector
 Blinker control circuit board (foam protected) with triple connector
 Red crimp-on female connector
 Tie wraps for securing circuit board and switch
 These instructions
Required tools:
 Phillips screwdriver.
 Pliers.

Terminology: Each diff-lock mechanism is controlled by 4 switches and a lot of vacuum line. Basically there is a vacuum line coming off of the brake booster line in the engine compartment. This feeds into a one way valve (to maintain vacuum in the diff-lock system) along with a vacuum reservoir under the car. There is a tee in this line that routes under the car and comes into the passenger compartment under the Diff-Lock Panel, which has one, two, or three Vacuum Dash Switches, controlling the Front-Diff, the Decoupler, and/or the Rear-Diff. The Vacuum Dash Switches control vacuum to the Vacuum Control Units on the differential or transmission. There is also a Contact Switch that senses engagement. Finally there is a Vacuum Sense Switch for the blinker unit included with the kit that you will be installing.

Detailed Instructions:

The Blinker Unit with Switch can be installed on either the front diff-lock or rear diff-lock system using the existing original front diff wiring harness installed originally on all Syncros. Everything just plugs together, except in order to connect to the rear diff dash bulb, we need to crimp on one connector for attachment to the Blinker Unit.

Make sure (before starting the procedure) that the diff-locks are working properly. Take your Syncro for a drive, and engage the diff-locks (on dirt, not pavement) and verify that they are functional and that the corresponding dash lights come on. Disengage the diff-locks and drive it into to your work area. Put your Syncro in neutral, and pull the diff-lock Vacuum Dash Switches. If either or both dash lights come on, then push the knobs back in, drive around the block, and try again. You are attempting here to get the diff-locks into "engaging" mode, not "engaged" mode (solid light). Now that the diff-lock splines are NOT engaged, shut off engine, set the emergency brake, and begin installation.

Pull off knob(s) on the existing diff-lock switch(es) by holding the shaft with smooth pliers (use tape on the plier's surfaces if necessary) and unscrewing the knob.

Remove the four screws securing the Syncro diff-lock panel onto the center console.
Now undo the gold metal bracket and pull out a ways to provide some working room.
Remove lower plastic shroud by pulling straight back; check clearance on top edge.

On the target diff-lock Dash Vacuum Switch locate the vacuum switch nipple that connects to the rubber tube that is closest to the knob. Separate the corresponding plastic line from the rubber line (usually in the shape of a J), and insert the separated ends into the opposite ends of the Y.

In the nest of wiring under the dash that runs to the lighted panel on the Dash Vacuum Switch unit, there are three unused connectors that are provided for the original front-diff blinker unit. The pair of brown-wires with single connectors on it should be connected securely to the two spade terminals on the Vacuum Sense Switch. The triple connector should be connected to connector that is attached to the foam-protected circuit board (it may take a bit of force to join the connectors securely - make sure similar colored wires are being connected.) Use one of enclosed tie-wraps to secure the Vacuum Sense Switch to the bundle of vacuum lines.

If you are connecting the blinker unit to the front diff-lock, you can skip the rest of this paragraph, as you are done with installation. For the rear diff-lock you will need to hook up the wire that connects to the rear diff-lock bulb. It's the center wire of the three wires that are connected to the circuit board. First, separate the single connector on this middle wire (white with a black stripe). Now you need to locate a section of wire in the original harness (close to the triple connector) that is blue with a brown stripe. Attach the red crimp-on female connector to this wire using a pair of pliers - you will know it is crimped properly when the little latch on it is closed. Attach the male connector on the middle wire to the crimped-on connector.

Now that your new blinker is installed, you should test it before putting the dash back together. Make sure that all the wire and vacuum lines are connected. Put the van in neutral, and start the engine. Let the vacuum build for up to a minute or so (may be necessary if your rubber vacuum lines have gotten a bit stiff and leaky with age). Then pull your dash switch. As we know (from the first step in this procedure) that it should be in "engaging mode", the light should be blinking. If so, we are done. If not, go back over the procedure and verify that everything is connected properly.

Replace bracket and panel being careful to not kink vacuum lines or pinch wiring. Start engine once again, and verify that dash light is blinking once the knob is pulled.
Enjoy your Syncro!!!!

Technical assistance or comments:
E-mail: (subject: blinker)